Combi boilers in large houses ?

5 Mar 2011
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United Kingdom

We are in the process of changing from oil boiler onto gas boiler and are considering all the options.

We've heard bad things about combi boilers and that they are no good in large houses (ours is large 4 bed detached)?

Plumbers have suggested installing an unvented system with a pressurised cyclinder water tank (we are moving water tank from airing cupboard into garage as well). We've had 2 plumbers round so far, 1 has suggested a combi boiler and the other said our house was too large?

Opinions please :)

Thanks in advance :)
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Large house usually means more than one bathroom and ensuites. Combis in general are suitable only for single point use, must have good water supply.
There are 35/40 kw boilers but won't run a shower and fill a bath satisfactorily at the same time. The heating side is no problem with a combi.
IMO fit an unvented cylinder together with a good boiler and it will meet all your needs.
A combi will be fine in any size house if there is only one person living there.

As the combi only supplies one hot outlet its the number of people which is the limiting factor. Two will interface with each other and ensure only one uses hot water at a time.

A good system uses a combi to supply the kitchen or one bathroom and an unvented cylinder for the rest.

Before designing a system the dynamic mains flow rate needs to be measured.

Thank you both for your replies :)

Sorry I should have explained more in my original post .... we are a family of 4 (2 kids) and have main bathroom with bath + separate shower, plus en-suite with shower, also a downstairs cloakroom.

Thank you :)
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You need a cylinder then! Or you will not get enough flow of hot water.

With an unvented cylinder, the total amount of water (hot and cold) emerging from all taps in the house at any time will not exceed the amount coming in through the water main. This even includes the electric shower if you have one. So you need to measure the flow by running the kitchen tap into a bucket and timing it (in litres per minute). If this flow is not enough, you will need to upgrade the water main (perhaps you have an old half-inch pipe) all the way from the roadside, or else have a water storage tank of some kind, or else move house to the bottom of a hill. You can use a pressure vessel to give a short-term pressure boost if you have a sealed cylinder. You cannot use a pump on the water main.

I am not a plumber but an old-fashioned householder, I actually am very pleased with the terrific flow at my bathtaps using a big tank in the loft. My system is newish with modern parts, but old-fashioned approach and I have an unusually tall house, which improves the pressure, especially at the downstairs shower.

Agile's suggestion of a combi plus a cylinder is very good, though not widely used as the installation cost is higher.
John has only described the open flow rate whereas you really need the dynamic flow rate which is often 50-70% of the open pipe flow.

The cost of a combi/unvented is lile more because all the competition is between the combi boilers so their price is little more than a heat only boiler.

Many installers dont even know how to do that though.

the advantages of having a seperate boiler and tank are not just in its performance but also in reliability and cost of maintenance.

I have no plumbing experience but I do have a combi boiler and if I had the option I would not put another one in. The central heating part of it is fine, or it's as good as the boiler you install but that goes for any system. The hot water however I am not impressed with we had to change the original combi boiler so put in a larger one, it was recommended by the Gas board to accomodate the amount of water used, but if someone is using the shower we try not to run the water downstairs, the drop in pressure has a significant impact. The system has been tested and we have good water pressure and I am told it should be fine but it is not and never has been. I have had mine a while now so there will be better ones on the market but given the choice I would not choose it.
I would need to replumb my whole house to change it now and that is not a realistic financial option but in your place I would not touch a combi boiler with the proverbial barge pole.
Good luck
Large size combis will happily do two showers but will struggle to fill two baths at the same time at a fair speed. As long as you don't have a demand for running two baths at full speed at the same time on a regular basis, you can save yourself £1000 - £2000 ( or more if the water needs upgrading ) if you go for a combi.
Nobody has ever said a 24 kW se combi didn't work, and a 40 kW condensing boiler has 90% of the output of two 24 kW se combis.

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